1) I hear that if the front diff whines when accelerating it is the pinion bearings. (and if the rear whines upon deceleration, it is the pinion). The wheel bearings normally don't have an acceleration/deceleration sound associated with them.
2) Pick up the diff fluid anywhere (Napa) buy you'll need to get the friction modifier from the dealership. Check out www.wjjeeps.com for the specifics on which gear oil you need.